A theoretical study of the pressure-induced dimerization of C60 fullerene

We present a theoretical study of the solid-state dimerization of C-60 fullerene, which occurs under pressure through [2+2] cycloaddition of double bonds. The possible crystal packings of (C-60)(2) molecules are calculated by minimization of the lattice energy with a bond charge intermolecular potential model proved successful in the previous C-60 studies. The set of dimer lattices that were derived from the fee lattice was used to construct the initial structures for minimization. The final structures found this way, although belonging to various Space-group symmetries, retain approximately fee arrangement of the constituting C-60 cages. On the other hand, the structure obtained from the dimer motif observed in the o-dichlorobenzene solvate of (C-60)(2) exhibits a:hcp-like C-60 arrangement. The more energetically stable hcp-type dimer seems not to form due to the high potential barrier separating the fee and hcp structures. The relative stability of the predicted structures changes significantly under pressure. Some of the dimer structures are remarkably similar to the respective theoretical monomeric and polymeric C-60 structures studied previously. On this basis we propose that the respective structures are connected states in the possible conversion paths from pristine C-60 to its various polymerization products. One of the dimer structures is geometrically favorable for the formation of the higher C-60 chain oligomers as well as of the infinite polymer.